http://articles.philly.com/2011-06-26/b ... l-gas-sand
just a bit:
Since the first well was hydraulically fractured in 1949, engineers have experimented with many different proppants, including broken walnut shells. Cheap and plentiful sand became the preferred proppant and now accounts for 88 percent of the market, according to research by industry consultants PropTester Inc. and Kelrik L.L.C.
Not all sand is suitable for proppant. Sand quarried from large Midwestern deposits has spherically shaped granules that flow smoothly into wells and leave sufficient gaps between the grains to allow oil and gas to flow.
But sand has limitations. In deeper wells, it is crushed under the tremendous weight of the earth, clogging the fractures and reducing production. Some producers strengthen the sand by coating it with resin.
A superior alternative is ceramic proppant, whose uniform shape and high strength boost gas production. Ceramics are produced from bauxite or kaolin clay, and they cost at least three times more than sand.
Nevertheless, ceramic producers cannot keep up with demand. Even Chinese imports cannot fill the market need.
Carbo Ceramics Inc., of Houston, the leading U.S. supplier, added two new plants this year. Its proppants sold for 35.1 cents per pound in the first quarter, up 12 percent over last year. Its stock price has doubled.